Tungsten-molybdenum trioxide catalyst



Patented Apr. 20, 1948 TUNGSTEN -MOLYBDENUM TRIOXIDE CATALYST Herrick R.Arnold, New Castle County, Del., as-

slgnor to E. I. du Pont de Nemours & Company, Wilmington, Del., acorporation oi Delaware No Drawing.

This invention relates to the production of a new catalyst and moreparticularly, it relates to a new catalyst for the catalytic oxidationof alcohols to aldehydes and ketones.

This application is a continuatlonein-part of application Serial No.322,932, flied March 8, 1940 (now U. 8. Patent 2,320,253, dated May 25,1943).

This invention has as an object to provide a new catalytic composition.Another object is to provide a new and improved oxidation catalyst.Still another object is -to provide a new catalyst which is effective inconverting alcohols to aldehydes and ketones and which is particularlyeffective in converting methanol to formaldehyde. Another object is toprovide methods for the preparation of these new and improved catalysts.Other objects will be apparent from the reading of the followingdescription of the invention.

These objects are accomplished by the following invention whichcomprises mixing or intimately associating an oxide of molybdenum withan oxide of tungsten in a mole ratio within the range of :1 to 1:10. Thecatalyst so obtained has been found to be particularly useful for thecatalytic oxidation of methanol to formaldehyde at temperatures withinthe range of about 225 to about 450 C. This catalyst is best prepared byacidifying intimately commin-gled finely ground wet mixtures of ammoniumsalts of tungsten and molybdenum.

The following examples are submitted by way of illustration and not aslimiting the invention.

Example I A catalyst containing the oxides of molyb-. denum and tungstenin approximately equimolar proportions is prepared by igniting a mixtureof the ammonium para-salts of molybdic and tungstic acids as follows:354 g. of ammonium paramolybdate, and 540 g. of ammonium para-tungstateare ground together to a dry,'powdered, mixture passing a 60-meshscreen. This mixture is then ignited to constant weight by heating at375 to 400 C. After cooling to room temperature, the dry powder iskneaded to a thick,

Application October 13, 1942, Serial No. 461,896

2 Claims. (01. 252-232) and passed continuously at the rate of 0.3 cubicfeet 'per hour. over cc. of the catalyst maintained at 260 to 310C. Theeflluent product is cooled and passed through water whereby an aqueoussolution of formaldehyde and unchanged methanol, containing an amountofformic acid equivalent to less than 0.1% of the methanol,

These conversions and yields are maintained without diminution for aperiod of about hours.

Example II A more active catalyst than the one described above, andcontaining the oxides of tungsten and molybdenum in the molar ratioWOa/MOOa =1.22, is prepared by the following improved method: 263.4 g.of ammonium para-molybdate and 430.2 g. of ammonium para-tungstate areground to a thick, uniform paste with 276 cc. of water. To the finelyground slurry there is added, with stirring, 350 cc. of a 42.5% solutionof nitric acid whereby a finely coagulated mixture of hydratedmolybdicand tungstic acids is precipitated in a. semi-gelatinous form. Theprecipitate is isolated by filtration and washed on the filter with 750cc. of water, dried at to C., and baked for 5 hours, starting at 210 C.and increasing the temperature to 315 0.,

after which the baking is continued at 315 to 320 C. for an additional 2hours. The resulting hard, porous, homogeneous mass is then sized to 4to 10 mesh granules. Sixty-five cc. of the catalyst, placed in asuitable converter, is heated to 258 C. A mixture of air and methanolvapor containing 8 to 10% of methanol is preheated and passedcontinuously over the catathan 0.1% of the methanol passed. When thereaction temperature is increased to 805 0., 9'7 to 100% conversion ofthemethanol to formaldehyde is obtained with no appreciable increase inthe formation of formic acid.

of this invention.

aasaeeo Example m The durability of life of the catalyst prepared inaccordance with Example II. is still further improved by introducinginto the method of preparation described in Example II the additionalstep of oxidizing the catalyst to its highest state of oxidation (namelyto a mixture of the catalytic oxides of molybdenum and tungstenconsisting substantially or mixtures of their trioxides'). This step isaccomplished by subjecting a catalyst prepared according tofixample II,above, to a heat treatment at temperatures of approximately 450 C. in astream of air or oxygen for periods of 14 to 48 hours whereby the loweroxides of molybdenum and tungsten are converted to the trioxides.Simultaneously the high temperature oxidation treatment has the effectof hardening the catalyst and making it more resistant to erosion anddisintegration during use.

When a mixture of air and methanol vapor containing 8 to 10% by weightof methanol is preheated and passed at the rate of 0.3 cu. it. perminute over 65 cc. of the catalyst, prepared as described-in thisexample, and maintained at 250 to 320 C., substantially 92% to 95% ofthe methanol fed into the catalyst chamber is recovered as formaldehydein the finished product over a period of about 1000 hours of continuousoperation.

Example IV Octyl alcohol is fed continuously at the constant rate of61.7 g. per hour into a vaporizing and preheating chamber where it isvaporized and mixed continuously with 2.9 times its weight of air. Theair-alcohol mixture is preheated to 300 C. and passed continuously atatmospheric pressure over 22.5 cc. the catalyst prepared as described inExample 11 heated to between 280 and 300 C. The time during which theairalcohol mixture remains in contact with the catalyst is about 0.26second. Under these conditions 66% of the octyl alcohol is oxidized tooctanal, and 95.2% of the alcohol processed is recovered as aldehyde andunchanged alcohol.

Similar yields of butyraldehyde, isobutyraldehyde, and cyclohexanone areobtained by oxidizing, respectively, n-butanol. iso-butanol. andcyclohexanol under the same conditions described above.

Although the foregoing examples have illustrated certain methods forpreparing the catalysts and the manner of their use in the oxidation ofalcohols to aldehydes and ketones it is to be understood that manymodifications and extensions'oi the invention may be made withoutdeparting from the spirit and scope thereof.

The catalysts contemplated in this invention comprise :1 to 1:10mixtures of the oxides of molybdenum and tungsten. These molybdicoxide-tungstic oxide catalyst compositions may be modified byincorporating therein a third oxide which acts as a promoting ormodifying agent. Examples of such oxides are the oxides of uranium,chromium, iron, manganese, copper,

cadmium, vanadium, etc. It is known that these two elements may exist inmany states of oxidaticn and all possible combinations or these oxidesare to be recognized as coming within the scope Although the preferredmixtures are those consisting oi and which contain these oxides inapproximately equimolar ratios, mixtures of the so-called "blue-f oxidesof tungsten andmolybdenum, as well as their dioxides and sesqui-oxides,are also contemplated within the scope of this invention.

The oxide catalysts may be prepared in a number of diiferent ways, asfor example: by simple mixing or grinding together of molybdic andtungstic anhydrides with sufiicient water to form a thick paste andsubsequently, drying, baking. and sizing the resulting product. A stillbetter method consists of igniting intimate mixtures of the ammoniumsalts of the metals. It is preferred, however, to coprecipitate hydratedmolybdic and tungstic acids in suitable proportions by acidifyingwetmixtures of the finely ground ammonium salts with an excess of nitric orhydrochloric acids or'mixtures of these acids, and drying and baking theresulting coagulated, semigeiatinous precipitate. A further step in thepreferred method consists in finally subjecting the catalyst totreatment in an oxidizing atmosphere at elevated temperatures,preferably in the range 350 to 450 C., to convert the oxide mixture tothe trioxides or highest state of oxidation, whereby a bright orange oryellow, porous, granular catalyst is obtained which has suflicientinherent strength to withstand erosion and other factors encounteredduring use which tend to cause disintegration of the catalyst.

Further improvements in physical strength of the catalyst may be made bybriquetting the cataiyst or by introducing binders such as colloidalsilica. In some cases it may be desirable or even preferable to applythe catalytic masses to carrier substances or surface-extending agentssuch as silica gel or kieselguhr, or the catalytically active compoundsmay actually be precipitated or otherwise formed within the pores of thecarrier or surface-extending substances.

Considering the oxidation of methanol to formaldehyde as a typicalexample of the use of the catalyst of this invention, the temperature ofthe catalyst may vary over a wide range dependent upon such otherfactors as time of contact and the particular composition of thecatalyst. While the temperaturesemployed may vary from about 225 C. toabout 450 'C., it is ordinarily preferred to operate within the range250 to 350 C.

The ratio of methanol to air used with these catalysts may also varyconsiderably, although usually the air is used in greater than thetheoretical quantity required to' oxidize the alcohol to formaldehyde.Mixtures containing 5 to 20% by weight of methanol in the form of vapormay be employed, but it is ordinarily preferred to use mixturescontaining 8 to 10% by weight of methanol. In place of methanol anyprimary or secondary alcohol may be converted to the correspondingaldehyde or ketone. The alcohol may be aromatic, aliphatic, or acycloaliphatic alcohol and one that is capable of vaporization.

Wide variations in the time of contact of the gases with the catalystmay be employed depending to some extent upon the temperature, comlowerlay-products, especially acid, than are possible with catalysts atpresent known to the art. As contrasted particularly with vanadium oxidecatalysts, the molybdenum oxide-tungsten oxide catalysts of thisinvention produce substantially acid-free formaldehyde. While thecatalysts of this invention may be used for the oxidation of anyalcohol, they are particularly useful in the conversion of methanol toformaldehyde.

The catalysts have the additional advantage that they possessconsiderable inherent strength so that they do not necessarily requirethe use of a support, and can he used for long periods withoutappreciable erosion or disintegration. Furthermore, although they arecatalytically active at relatively low temperatures, they are quiteresistant to high temperatures of the order of 450 to 550 C. or higher,where many catalysts ordinarily sinter and become inactive. They arecomparatively insensitive, also, to the usual catalyst poisons such assulfur and its compounds.

It is apparent that many widely different embodiments of thisinventionmay be made without departing from the spirit and scope thereof andtherefore it is not intended to be limited except as indicated in theappended claims. 1

I claim:

1. An oxidation catalyst composition consisting of an intimate mixtureof the trioxides of tungsten and molybdenum in which the mole ratios ofthe trioxides varies from :1 to 1:10, said catalyst being porousand-granular in nature and of bright orange-yellow color.

2. An oxidation catalyst composition as defined in claim 1 characterizedin that the trioxides are present in equimolecular proportions.

HERRICK R. ARNOLD.

REFERENCES c'rrnn The following references are of record in the file ofthis patent:

UNITED STATES PATENTS France Jan. 26, 1933

